Originally Posted by bmcer

I like the Take-A-Look mirror. It mounts to your glasses, but there's an adapter sold separately that also allows a helmet mount. Since I always wear glasses of one sort or another, I've not tried the helmet mount option. I prefer glasses/helmet mounts because I can take a quick look to the rear without changing my head position as much as I need to with bar-end mirrors.

Originally Posted by Jim from Boston

I use both left and right rearview mirrors, in my case Take-a-Look eyeglass mounted ones. I got the idea from a cycling companion who used only a right hand mirror. The additional right hand mirror affords a pretty good rearward view, but is particularly useful:

1. Riding on the left-hand side of a one-way street
2. Riding in the middle or left lanes of a two-way thoroughfare
3. In a rotary
4. On a curved road to the right
5. When passing entrance/exit ramps from a freeway, with the right hand mirror, I can view the ramps to my right, and stay wide of them, while watching upcoming traffic on my left, all while almost continuously looking straight ahead
6. When the sun is directly behind, usually one mirror can be positioned away from the glare of the sun
7. When wearing a backpack, usually one mirror has a less-obstructed view over my shoulder.

My main argument for a mirror, particularly in the urban environment is summarized by Jim’s Law of the Road: “No matter how well paved or lightly-traveled the Road, a vehicle is likely to pass you on the left as you encounter an obstacle on the right.”

One other situation where hearing is ineffective, even on rural roads, occurs when being passed by one car, and I'm never sure that another one is following the first. No problem with a mirror...